Electric furnace and electric resistance element therefor



Feb. 24, 1931. H. J. McCAULEY' ELECTRIC FURNACE AND ELECTRIC RES IS'IANCE ELEMENT THEREFOR Filed July 31. 1929 INVENTOR ATTORNEYS Patented Feb. 24, 1931 UNITED STATES PATENT OFFICE ELECTRIC FURNACE AN D ELECTRIC RESISTANCE ELEMENT THEREFOR Application filed July 81, 1929. Serial 110.88%807.

This invention relates to electric furnaces and electric resistance elements therefor. The principal object of the invention is the provision of certain improvements in electric furnaces of the resistance type. Thus, the invention contemplates the provision of an improved electric resistance element as well as the provision of an improved electric furnace construction- The resistance or heating elements heretofore used in electric furnaces have a comparatively short useful life due to failure or burning-outof the element at places of localize-d heating. Such localized heating is 5 most frequently occasioned by an excessive current density, by inadequate or improper heat radiation, or by mechanical defects or Weaknesses in the element. The renewal of a burned-out element necessitates interruption of the furnace operation, and often cooling down of the furnace, besides frequently involving considerable labor and trouble. Various types of resistance elements have heretofore been devised for ameliorating these difliculties, but the progress of the electric resistance furnace is still greatly impeded by the frequency of heating element failures.

The improved resistance element of my in- 80 vention is of such construction that the heat developed therein is effectively radiated therefrom, with substantially no localized heating. The element has an extremely long useful life, has great mechanical strength,

occupies a relatively small s ace, and is applicable to any surface in the urnace or similar heating appliance. The element is capable of use with commercial volta es, and constitutes a non-inductive electricload. It is readily inserted in and removed from the furnace thereby making renewals easy.

The improved electric furnace of the invention is characterized by its simplicity of construction and its remarkable thermal eiliciency. The furnace differs from the heretofore customary types of electric resistance furnace in that it has no refractory liner or inner wall, the inner Wall or lining of the furnace of the invention consisting simply of heat-insulating material. As a consequence of this construction, a remarkably short time is required for heating the furnace to its operatingtemperature- In the furnace of the invention, the element-holding. keys or members are individually removable with'out disturbing either the furnace walls or the resistance elements. While the improved furnace structure is particularly adapted to use the resistance elements of the invention, other practical] fiat types of resistance elements may be a vantageously used.

The patentably novel features of'the invention are definitely recited in the appended claims. The construction and unique character of the improved electric furnace and resistance element of the invention will be understood from the following description taken in conjunction with the accompanying drawing; in which Fig. 1 is a sectional elevation'of an electric furnace embodying the invention;

Fig. 2 is a plan view of the improved resistance element of the invention;

Fig. 3 is a pers ective view of the resistance element below tlie section line 33 of Fig. 2; and

Fig. 4 is a detailed view of the manner of assembling the element-holding members in the furnace structure.

Referring first to Figs. 2 and 3 of the drawing, it will be noted that the resistance element is in the form of a sheet or plate of conducting material providing a zig-zag or sinuous path for the passage of the electric current. The zig zag current path results from a series of s aced incisions 5 extending from one edge of the sheet to near but not-- entirely to the opposite edge of the sheet and alternating with similarly spaced incisions 6 extending from said opposite edge to near but not entirely to the first mentioned edge. The incisions 5 and 6 are preferably uniformly s aced apart, whereby the strips of material between adjacent incisions are of uniform width. These strips of material 7 provide a plurality of transverse conducting paths electrically connected in series at their ends by the integral portions 8 of the sheet. The inner or closed end of each incision terminates at such a distance from the adjacent edge of the sheet that the current density in the portions 8 is no greater and preferably less than the current density in the strips 7. Satisfactory results are obtained where the incisions 5 and 6 terminate at a distance from the adjacent edge of the sheet approximately equal to or preferably slightly greater than the distance between adjacent incisions.

The strips of material 7 have their edges so bent that the opposite faces of each strip are concave and convex respectively. The edges of the strips 7 are similarly bent so that the concave sides of all the strips are on the same face of the sheet, and likewise the convex sides of all the strips are on the same but opposite face of the sheet. The portions 8 of the sheet are substantially fiat and lie in substantially the same plane. the strips 7 is such that the crests 9 of the convex sides thereof are in substantially the same plane as the flat portions 8 of the sheet. Thus, when the element is positioned with the convex faces of its strips 7 adjacent a plane surface, such as a flat furnace wall, the crests 9 and the fiat portions 8 will lie against and practically in contact with the plane surface. Thebending of the strips 7 in the manner described very substantially strengthens the sheet as well as permits most effective radiation of the heat generated in the element, as more fully described hereinafter.

The resistance element is 1preferably fabricated from a sheet of a nic el-chromium alloy, such as nichrome, although other resistance materials are satisfactory for the purpose. The incisions 5 and 6 are cut in the sheet and the strips 7 are bent without removing any material, so that the entire area of the. original, unformed sheet is available and utilized as an electric conductor. The sheet or plate from which the element is formed may be of any appro riate thickness and area andthe width of t e strips 7 may be appropriately varied, to suit differing voltage, load and installation conditions.

In fabricating the element, the incisions 5 and 6 are preferably formed by appropriate dies without removal" or waste of stock. Thus, the sheet may be progressively fed or advanced through an appropriate machine having one or more pairs of dies adapted to simultaneously cut one or more pairs of incisions 5 and 6. The'bending of the strips 7 is similarly performed by appropriate dies or punches. It will be noted that the bending of the strips 7 results in a separation of the material of adjacent strips, so that an elongated space 10 is provided between the strips. At the same time, the bending operation slightly slightly increasing sheet, and providing between the adjacent stretches the strips 7, thereby the overalllength of the definite spaces or slots flat portion 8 which beyond the ends of the strips 7 Moreover, the bending of positively separate and electrically insulate these ad acent flat portions. The incisions 5 and 6, which in fabricating the resistance element start as merely a severance or cut in the sheet, assume the form, in the finished element, of an elongated space 10 terminating at one end in a slot or space. These slots or spaces, resulting from the bending of the strips 7, are of suflicient width to take up linear expansion of the element when heated to its operating temperature, Thus, the fiat portions 8 of the element, in addition to facilitating the ready insertion and removal of the element in and from the furnace, act to eliminate elongation of the element through expansion, since any expansion in the flat por-. tions is taken up in the slots and not in the overall length of the element.

In Fig. 1 of the drawing, I have shown an electric furnace in which the resistance elements of the invention are assembled. The furnace is of the tunnel type, comprising an outer steel skeleton or frame 11, an outer brickwork structure 12, an inner heat-insulating lining 13, and a heat insulating roof or arch 14. Elementshqlding keys or members-15, or porcelain or equivalent electric insulating material, are assembled in the lining 13. These members have. a fiat, horizontally disposed body portion embedded for the most part in the lining 13. At their inner ends, the members terminate in a web or flange 16 suitably spaced from the wall or face of the lining. At their outer ends .the members have a cruciform opening '17 in which the head 18 of a bolt rod 19 is engaged. The rod 19 extends through the furnace wall and is held in position by a nut 20 on its outer threaded end. The members 15 are thus securely held inpositiomwhile permitting convenient replacements thereof when necessary, without disturbing the furnace wall or lining.

The resistance elements (R) are held substantially flat against the lining by the members 15. The fiat end ortions 8 of the ole-- ments are adapted to ide in the spaces between the lining and the flan es 16,and may be. conveniently assembled in or removed from the furnace by merely sliding them along the. face of the lining between adjacent confining flanges. The elements R are positioned with the convex faces of the strips 7 adjacent the lining, whereby the concave faces of the strips are directed towards the furnace chamber to focus and radiate the maximum amount of heat into the same. At the same time, this arrangement of the elements avoids any rear air pockets or substantially closed spaces in which objectionable local heating or heat concentration might take place. Moreover, since the flat portions 8 and the create 9 of the strips 7 lie in substantially the same lane and adjacent the surface or face the 13,110 edges or projections impede the sliding of the element across the face of the lining.

The heating elements cover substantially the entire surface or face of the side and bottom walls of the furnace chamber. If necessary or desirable, the elements may similarly cover the roof and end walls of the chamber. In the drawing, I have shown a slightly modified form of element-holding key 15' on the bottom of the furnace chamber. These keys 15! are bolted or otherwise appropriately secured to the bottom of the furnace chamber, and are provided with recesses on each side in which the flat portions 8 of the element are adapted to engage and to be thereby held in position. In the drawing, a Work-supporting table 21 is shown mounted on the bottom of the furnace chamber above the resistance elements, but it is to be understood that the work to be furnaced may be supported in any other appropriate manner.

The resistance elements are free to expand and contact along the confining and sup porting insulating guides or members without imposing any additional strain on the members and without objectionable effect on either the mechanical or heating properties of the elements. The strain of supporting the elements between the holding members is so slight that the furnace lining may be made of relatively soft or mechanically weak heatinsulating material, such for example as silo-cel brick and the like. The necessity of a refractory liner or wall is thus dispensed with, and the entire inner wall or lining of the furnace may be built of material particularly selected for its heat-insulating properties. This results in a furnace structure having a relatively short time lag for furnace saturation. For example, furnaces embodying th invention may be heated from room tempera ture to 1600 F. in 30-60 minutes, and are capable of operating continuously at a temperature of 2000 F.

The improved resistance element of the invention has an extremely long useful life due to the low watt density on the element per square inch of radiation surface, producing a verv small temperature differential between the element and the work. The element occupies relatively small space, and is applicable to all surfaces in the furnace, and is a non-inductive electric load. The element is extremely easy to remove from the furnace or renew, and it has great mechanical strength and is not restricted as to size. It is possible to arrange the electrical circuits with these elements as desired and also to localize the application of heat. Moreover, the element is flexible, non-fragile and ductile. It has an extremely high resistance and is useable on commercial voltages at a low current density and at a low watt rating, producing an intense heat with a minimum expenditure of power. Electric furnaces embodying the resistance elements of the invention are charac terized by the long useful llfe of the elements and by the relatively short time required toheat the furnace to its operating temperature. \Vhile the improved resistance element of the invention is particularly adapted for use as a heating unit in an electric furnace or the like, it is susceptible of general use as an electric resistance unit, as, for example, in rheostats and other electrical equipment.

I claim:

1. An electric resistance element comprising a sheet of conducting material providing a zig-zag electric current path, the main portion of the sheet consisting of substantially parallel units of concavo-convex configura tion having their ends electrically connected in staggered pairs by substantially flat portions of the sheet.

2. An electric resistance element comprising a sheet of conducting material providing a zig-zag electric current path in consequence of a plurality of spaced incisions extending alternately from opposite edges of the sheet to near but not to the respective opposite edges, the strips of material between adjacent incisions being of concavo-com ex configuration While the portions of said sheet beyond the ends of said strips are substantially flat.

3. An electric resistance element comprising a sheet of conducting material having a plurality of spaced incisions extending alternately from opposite edges of the sheet to near but not to the respective opposite edges, the edges of the strips of material between said incisions being so bent that the opposite faces of each strip are concave and convex respectively, the portions of the sheet beyond each end of said strips being substantially fiat and in substantially the same plane as the crests of the convex faces of said strips.

4. An electric resistance element comprising a sheet of conducting material having a plurality of spaced incisions extending from one edge of the sheet to near but not entirely to the opposite edge of the sheet and alternating with similarly spaced incisions extending from said opposite edge to near but not entirely to said first mentioned edge, the strips of material between the alternate incisions having their adjacent edges bent away from one another thereby mechanically strengthening said strips, the portions of the sheet be yond each end of said strips being substantially fiat.

5. An electric resistance element comprising a sheet of nichrome metal having a plurality of spaced incisions extending alternately from opposite edges of the sheet to near but not to the respective opposite edges thereby forming a zig-Zag electric current path, the strips of material between adja cent incisions being of concavo-convex configuration while the portions of said sheet beyond the ends of said strips are substantiallyflat.

6. An electric resistance element comprising a sheet of conducting material having a plurality of conducting strips of concavoconvex configuration electrically connected in series by substantially flat integral end portions.

7. An electric resistance element comprising a sheet of conducting material having a plurality of conducting strips of concavoconvex configuration electrically connected in series b substantially flat integral end portions, t e crests of the convex faces of said stripsand said flat end portions being in substantiallythe same plane.

8. An electric resistance element comprising a sheet of conducting material having a plurality of conducting strips of concavoconvex configuration electrically connected in series by substantially flat integral end portions, the effective width of 'said end portions being at least as great as the effective width of said strips.

9. In an electric furnace, the combination with a fiat wall of spaced guides secured thereto, and a resistance element comprising a plurality of conducting strips of concavo-convex configuration electrically connected in series by substantially flat integral end portions mounted in said guides with the.

convex sides of said strips adjacent said wall and the flat end portions engaging said guides. I

10. In an electric furnace, an electric resistance element comprising a plurality of conducting strips of concavo-convex configuration mounted on a wall of the furnace with the crests of the convex faces of said strips in proximate contact with said wall.

11. In an electric furnace, an electric resistance element comprising a plurality of conducting strips of cancavo-convex configuration electrically connected in series by substantially flat end portions mounted on a wall of the furnace with the crests of the convex faces of the strips and the flat connecting end portions in proximate contact with said wall.

12. In an electric furnace, spaced guides mounted on a wall of said furnace, and an electric resistance element comprising a plurality of conducting strips of concave-convex configuration electrically connected in series by substantially flat end portions loosely mounted in said uides with the crests of the convex faces of-t e strips and the flat connecting end portions in proximate contact with said furnace wall.

13. In an electric furnace, the combination with a furnace wall having a lining of high heat-insulating capacity of spaced elementsupportin members partly embedded in said liming extending through the furnace wall, adentd erbariorly thereof and having 

